This invention relates to voltage regulator circuits. More particularly, this invention relates to voltage regulator circuits for dual supply regulators in which one supply provides output current to the load and the other supply provides control power.
Voltage regulator circuits require a minimum voltage differential between the input supply voltage and the regulated output voltage in order to function properly. This voltage differential is known as the dropout voltage of the regulator. For a given supply voltage, the dropout voltage of the regulator limits the maximum regulated voltage which can be supplied to the load. Conversely, for a given output voltage, the dropout voltage determines the minimum supply voltage required to maintain regulation.
One potential deficiency in known voltage regulators is the tendency for such regulators to consume a larger percentage of the supplied power as the output voltage decreases. For example, a voltage regulator providing a 10 volt output with a 1 volt dropout results in a ten percent power loss, while an output of 2 volts (i.e., an output voltage) with the same 1 volt dropout results in a fifty percent power loss. However, there have been increasing requirements for voltage regulators to operate at lower and lower voltages (e.g., the voltage at which microprocessors are powered has continued to fall from 5 volts to below 3 volts). As microprocessor voltages continue to fall, their clock speeds and supply currents are increasing. Thus, low dropouts are required to efficiently supply modern microprocessor regulated voltage inputs.
Another consideration in integrated circuit voltage regulators is the utilization of an NPN transistor as the output transistor to take advantage of the smaller die size (versus a PNP transistor). Those regulators that utilize NPN output transistors are limited to a minimum dropout voltage of about 1 volt (i.e, approximately equal to V.sub.BE of the NPN transistor+V.sub.CESAT of a PNP that drives the base of the NPN transistor). Further, in many integrated circuit voltage regulators where the substrate is connected to the output, the control circuitry operates using the same supply as that which supplies the output power. As such, the control circuitry operates based on the difference between the input and the output of the device (i.e., the input-to-output differential). Thus, the dropout voltage is limited by the minimum operating voltage of the control circuitry, which is typically designed to operate down to about a 1 volt input-to-output differential.
A voltage regulator having a low dropout voltage is therefore capable of providing a regulated output voltage at a lower supply voltage than can a voltage regulator having a higher dropout voltage. A low dropout voltage regulator can also operate with greater efficiency, since the input/output voltage differential of the regulator, when multiplied by the output current, equals the power dissipated by the regulator in transferring power to the load. For at least these reasons, a voltage regulator circuit having a low dropout voltage has many useful applications, and can improve the performance and reduce the cost of other circuits in which the regulator circuit is used.
One known way of achieving a low dropout voltage is to provide individual supplies for the control circuitry and for the output power (i.e., dual supply regulators). The implementation of dual supply regulators, however, presents its own set of problems. One of the most severe problems is that of sequencing the individual supplies during power-on. As the two supplies are independent, it is often difficult to control which supply turns on first, thus resulting in a potentially dangerous situation. For example, if the regulator circuits do not function in a controlled manner regardless of which supply turns on first, the regulator may power up in a non-regulating condition or the regulator may latch. In either condition, the results may be severe, potentially resulting in destructive damage to the load (which could be an expensive microprocessor) caused by an output that goes to an unregulated high. One known solution to the problems associated with supply sequencing is to require a large minimum load (i.e., a large enough load to sink all of the drive current so that the output does not go to an unregulated high). This solution, however, is often not acceptable because it significantly degrades efficiency at lighter loads.
In view of the foregoing, it would be desirable to provide circuits that efficiently achieve a very low dropout voltage of less than 1 volt.
It would further be desirable to provide dual supply regulator circuits that efficiently provide a very low dropout voltage irrespective of supply sequencing during power-on.